As an in-vehicle display device, a liquid crystal display device capable of reducing the weight and using a battery equipped in a vehicle without change has been paid attention. There is a need of lowering brightness of a background display area and a dark display area in order to enhance expensive looking. A normally black type liquid crystal display unit has been developed which uses a light emitting diode as a light source of a backlight to emit light generally at a single wavelength and increase a contrast dramatically only in this wavelength range.
A display device utilizing a vertical alignment mode (VA mode) liquid crystal cell has been paid attention as a liquid crystal display device capable of realizing high-quality normally black display independent from an emission wavelength of a backlight. In a VA mode liquid crystal cell, liquid crystal molecules are vertically aligned relative to a substrate surface when voltage is not applied or off-voltage is applied (collectively called “in an off state” in some cases). “Vertically aligned” does not mean that a direction of a director of liquid crystal molecules is strictly vertical to the substrate surface, but it means that the director of liquid crystal molecules is aligned along a direction nearly vertical to the substrate surface as compared to that the director is inclined from a substrate normal direction when voltage is applied (called “in an on state” in some cases). This liquid crystal cell is disposed between two polarizers in approximately cross-Nichol configuration to constitute a liquid crystal display unit and realize normally black display.
The optical characteristics of a vertical alignment mode liquid crystal cell are approximately isotropic when observed along a substrate normal direction, and the optical characteristics of a liquid crystal display unit are influenced by the optical characteristics of polarizers in cross-Nichol configuration. A transmittance becomes therefore very low, and a high contrast can be realized relatively easily. However, when observed along an oblique direction, optical leak occurs in the black display state. This is because birefringence occurs in the liquid crystal layer, and the transmission axes of two polarizers shift from the perpendicular relation. In order to suppress a contrast, when observed along an oblique direction, from being lowered, the following various methods have been proposed.
JP-SHO-62-210423 discloses a liquid crystal display unit having a viewing angle compensator having negative uniaxial optical anisotropy or negative biaxial optical anisotropy inserted at one or both positions between a liquid crystal cell and two polarizers. The positive optical anisotropy of a liquid crystal cell in a thickness direction is compensated by the viewing angle compensator having the negative optical anisotropy in a thickness direction. The “viewing angle compensator having the negative biaxial optical anisotropy” means a viewing angle compensator having a relation of nx>ny>nz where nx, ny and nz are x-, y- and z-components of a refractive index in which an x-axis is a slow (lag phase) axis direction in an in-plane of a substrate or film, a y-axis is a fast (advance phase) axis direction and a z-axis is a thickness direction. The “viewing angle compensator having the negative uniaxial optical anisotropy” means a viewing angle compensator having a relation of nx=ny>nz. JP-A-2000-131693 discloses the effective conditions for an in-plane phase difference and an arrangement of the azimuth of an in-plane slow axis of a viewing angle compensator having biaxial optical anisotropy.
A viewing angle compensator having negative uniaxial optical anisotropy is called a “negative C plate”. A viewing angle compensator having negative biaxial optical anisotropy is herein called a “negative biaxial film”. A viewing angle compensator having positive uniaxial optical anisotropy with its slow axis being oriented in an in-plane direction, i.e., an optical film having a relation of nx>ny=nz, is called a “positive A plate”. The positive A plate can be considered as a special example wherein the refractive indices ny and nz of a negative biaxial film are equal to each other.
JP-A-2000-39610 discloses a method of using an approximately half-wavelength plate having biaxial optical anisotropy, and a negative C plate. With this method, since the half-wavelength plate is required to provide a phase difference of approximately a half wavelength when observed along any direction, a half wavelength plate having positive biaxial optical anisotropy is required in practical uses. However, it is difficult to realize a half-wavelength plate having positive biaxial optical anisotropy.
JP-A-2003-262869 discloses a method of using a combination of a negative biaxial film and a negative C plate. With this method, an in-plane retardation of the biaxial film is limited to 190 nm or smaller, and a retardation of a liquid crystal layer is limited to 200 to 500 nm. A retardation of a liquid crystal layer is represented by Δnd where Δn is refractive index anisotropy of liquid crystal material and d is a thickness of the liquid crystal layer.